libcurl-multi(3) -- Linux man page

NAME

libcurl-multi - how to use the multi interface

DESCRIPTION

This is an overview on how to use the libcurl multi interface in your C
programs. There are specific man pages for each function mentioned in
here. There's also the libcurl-tutorial(3) man page for a complete
tutorial to programming with libcurl and the libcurl-easy(3) man page
for an overview of the libcurl easy interface.

All functions in the multi interface are prefixed with curl_multi.

OBJECTIVES

The multi interface introduces several new abilities that the easy interface
refuses to offer. They are mainly:

1. Enable a "pull" interface. The application that uses libcurl decides where
and when to ask libcurl to get/send data.

2. Enable multiple simultaneous transfers in the same thread without making it
complicated for the application.

3. Enable the application to select() on its own file descriptors and curl's
file descriptors simultaneous easily.

ONE MULTI HANDLE MANY EASY HANDLES

To use the multi interface, you must first create a 'multi handle' with
curl_multi_init(3). This handle is then used as input to all further
curl_multi_* functions.

Each single transfer is built up with an easy handle. You must create them,
and setup the appropriate options for each easy handle, as outlined in the
libcurl(3) man page, using curl_easy_setopt(3).

When the easy handle is setup for a transfer, then instead of using
curl_easy_perform(3) (as when using the easy interface for transfers),
you should instead add the easy handle to the multi handle using
curl_multi_add_handle(3). The multi handle is sometimes referred to as a
'multi stack' because of the fact that it may hold a large amount of easy
handles.

Should you change your mind, the easy handle is again removed from the multi
stack using curl_multi_remove_handle(3). Once removed from the multi
handle, you can again use other easy interface functions like
curl_easy_perform(3) on the handle or whatever you think is necessary.

Adding the easy handle to the multi handle does not start the transfer.
Remember that one of the main ideas with this interface is to let your
application drive. You drive the transfers by invoking
curl_multi_perform(3). libcurl will then transfer data if there is
anything available to transfer. It'll use the callbacks and everything else
you have setup in the individual easy handles. It'll transfer data on all
current transfers in the multi stack that are ready to transfer anything. It
may be all, it may be none.

Your application can acquire knowledge from libcurl when it would like to get
invoked to transfer data, so that you don't have to busy-loop and call that
curl_multi_perform(3) like crazy. curl_multi_fdset(3) offers an
interface using which you can extract fd_sets from libcurl to use in select()
or poll() calls in order to get to know when the transfers in the multi stack
might need attention. This also makes it very easy for your program to wait
for input on your own private file descriptors at the same time or perhaps
timeout every now and then, should you want that.

A little note here about the return codes from the multi functions, and
especially the curl_multi_perform(3): if you receive
CURLM_CALL_MULTI_PERFORM, this basically means that you should call
curl_multi_perform(3) again, before you select() on more actions. You
don't have to do it immediately, but the return code means that libcurl may
have more data available to return or that there may be more data to send off
before it is "satisfied".

curl_multi_perform(3) stores the number of still running transfers in
one of its input arguments, and by reading that you can figure out when all
the transfers in the multi handles are done. 'done' does not mean
successful. One or more of the transfers may have failed. Tracking when this
number changes, you know when one or more transfers are done.

To get information about completed transfers, to figure out success or not and
similar, curl_multi_info_read(3) should be called. It can return a
message about a current or previous transfer. Repeated invokes of the function
get more messages until the message queue is empty. The information you
receive there includes an easy handle pointer which you may use to identify
which easy handle the information regards.

When all transfers in the multi stack are done, cleanup the multi handle with
curl_multi_cleanup(3). Be careful and please note that you MUST
invoke separate curl_easy_cleanup(3) calls on every single easy handle
to clean them up properly.

If you want to re-use an easy handle that was added to the multi handle for
transfer, you must first remove it from the multi stack and then re-add it
again (possibly after having altered some options at your own choice).